Microwave imaging systems are currently used in a variety of applications for detecting the presence and location of buried objects based on the reflections of microwave radiation. These systems typically illuminate a composite having specific dielectric properties with electromagnetic (EM) waves, which penetrate into the material where they interact with its interior. The waves are then reflected back from the material's subsurface defects and their properties are monitored to convey information about the composite at hand. Since most dielectric materials such as clothing, paper, and plastic are nearly transparent over the spectral band of microwave radiation, microwave imaging shows promising results in a variety of applications ranging from security inspection systems to ground penetrating radars. Moreover, as it is non-ionizing at moderate power levels, microwave radiation has the advantage over a number of other sources of radiation of posing no known health risk. As a result, microwaves appear well suited to biological sensing applications. More specifically, microwave imaging overcomes several limitations of common early-stage breast cancer detection systems such as mammography, ultrasound and Magnetic Resonance Imaging (MRI). X ray mammography, for example, exposes patients to low levels of ionizing radiation and often results in false diagnostic. It is also uncomfortable as breast compression is often required to reduce image blurring and to create tissue uniformity.
At microwave frequencies a significant dielectric contrast between normal and malignant tissue is found. Typical ex-vivo breast tissue, for instance, has a relative dielectric constant of about 10, while a malignant tumour has a relative dielectric constant in the range of 45-55. As a result, less attenuation and reflection are expected from normal than from malignant tissues and the tumour microwave scattering cross-section is much larger than that of a normal breast tissue with the same size. Moreover, healthy, fatty tissues are relatively translucent to microwaves since attenuation in normal breast tissue is low enough to make signal propagation through even large breast volumes quite feasible. Microwave imaging is therefore a safe, comfortable, sensitive and accurate method, which is attractive for early breast cancer detection. Existing approaches use antennas to illuminate the breast area with an ultra-wide band pulse and detect the energy reflected from or transmitted through the breast, from which images are formed to indicate the locations of strongly scattering objects. Additionally, several antennas are typically arranged to form an array, which enables scanning of a number of locations surrounding the breast.
A plurality of antenna designs has been disclosed in the art for microwave imaging applications. However, these antennas have various drawbacks such as their large size and non-planar structure, which make them difficult to use as a base for an antenna array of several elements. Bowtie antennas, for example, are often favoured for their remarkable broadband properties. The main concern however is the reflection from the antenna ends, which can be minimized with variable resistive loading. However, parameters required for the suggested design, such as a specific variation of the resistive loading and the equivalent surface resistance of the antenna, challenge its practical implementation. In addition, the antenna's overall efficiency could be significantly lowered by the high value of the equivalent surface resistance.
What is therefore needed, and an object of the present invention, is a planar and ultra-compact antenna for use in microwave imaging systems, where the antenna has constant resistive loading and reflection from the antenna ends is minimized through a change in the antenna geometry.